Acids and Bases React

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Chapter 15

• Acids and Bases React

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Section 15.1Acid and Base Reactions

• The reaction of an acid with a base is called a
  neutralization reaction.
• In most cases, the products are water and a salt.
  
• A salt is the negative part of an acid and the
  positive part of a base.

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Strong Acid Strong Base

• Remember that strong acids and bases dissociate
  completely when ionize in water.
• HCl(aq) ? H(aq) Cl-(aq)
• NaOH (aq) ? Na (aq) OH- (aq)

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Ionic Equation

• Ionic compounds primarily exist as ions in
  solution and are shown as ions.
• Ex. H (aq) Cl- (aq) Na (aq) OH- (aq) ?
• Na (aq) Cl- (aq) H2O (l)
• Notice that the acid and base are shown
  completely ionized. Water only ionizes slightly
  and is shown as a molecule.

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Spectator Ions

• Notice in the previous ionic equation that Na
  Cl- are present in the reactants and the
  products.
• They are called spectator ions (ions that are
  present but do not participate in the reaction)

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Net Ionic Equations

• Simplify the following math equation.
• y x 3 x2 x 3
• y x2
• A net ionic equation has the ions common to both
  sides of the equation removed.
• H (aq) Cl- (aq) Na (aq) OH- (aq) ?
• Na (aq) Cl- (aq) H2O (l)

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Net Ionic Equation

• H (aq) OH- (aq) ? H2O (l)
• The net ionic equation is hydronium ions
  reacting with hydroxide ions to produce water.
• This is always the net ionic equation for a
  strong acid and a strong base.

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Sample Problem 1Pg 521

• Write the overall (balanced), ionic, and net
  ionic equation for the reaction of sulfuric acid
  and potassium hydroxide.
• Overall
• H2SO4 (aq) 2 KOH (aq) ? K2SO4 (aq) 2 H2O (l)

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Ionic

• 2H (aq) SO42- (aq) 2K (aq) 2OH- (aq)
• ? 2K (aq) SO42- (aq) 2H2O (l)

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Net Ionic

• 2H 2OH- ? 2H2O (l)
• H OH- ? H2O (l)

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Practice Problems

• Work Problems 1-4 on Pg 521.

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The pH Perspective

• Acid-Base reactions are called neutralization
  reactions because the hydrogen ion from the acid
  reacts with the hydroxide from the base to form
  water, which is neutral.

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Strong Acid Weak Base

• Remember that weak bases only ionize partially in
  water.
• Al(OH)3 ? Al3 3OH-
• Consider the following reaction
• 3HBr Al(OH)3 ? AlBr3 3H2O
• Write the ionic equation.

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Strong Acid Weak Base

• 3H 3Br- Al(OH)3 ? Al3 3Br- 3H2O
• Remove spectator ions for Net Ionic Equation
• 3H Al(OH)3 ? Al3 3H2O
• A strong acid and NH3
• HCl NH3 ? NH4Cl
• H Cl- NH3 ? NH4 Cl-

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Practice Problems

• Pg 525 5-7
• Strong Acids react with weak bases.

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A Broader Definition

• Every reaction between an acid and a base does
  not result in a neutral solution.
• BrØnsted-Lowery definition an acid is a
  substance that donates, or gives up, a hydrogen
  ion (proton) in a chemical reaction.

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A Broader Definition

• A base is a substance that accepts a hydrogen ion
  in a chemical reaction.

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Defining Acids and Bases by H Transfer

• HCl H2O ? H3O Cl-
• acid base
• H3O NH3 ? NH4 H2O
• acid base
• H2O NH3 ? NH4 OH-
• acid base

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H NH3 ? NH4

• Ammonia does not create a hydroxide and would not
  be considered a base.
• Ammonia is definitely a Bronsted-Lowery base. It
  accepts a hydrogen ion in the reaction.

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It Takes Two to

• Transfer For every acid (an H donor), there
  must be a base (an H acceptor).
• Water can act as an acid or a base depending on
  what else is in solution.

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Water is not Required

• Most reactions of acids and bases occur in water.
• H transfer does not require water. HCl gas and
  NH3 gas will react without water being present.

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Weak Acid Strong Base

• HC2H3O2 NaOH ?
• NaC2H3O2 H2O
• HC2H3O2 Na OH- ?
• Na C2H3O2- H2O
• HC2H3O2 OH- ? C2H3O2- H2O

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Problems

• 8-10 Pg 530

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Applications of Acid-Base Reactions

• Section 15.2

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Buffers to Regulate pH

• Buffer- a solution that resists change in pH.
• Buffers contain ions that react with H and OH-
• Buffers are a weak acid and one of its salts, or
  a weak base and one of its salts.

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NH3 NH4Cl

• If acid is added, NH3 reacts with the H
• NH3 H ? NH4
• If base is added, the NH4 ion from the salt
  reacts with OH-
• NH4 OH- ? NH3 H2O

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HC2H3O2 NaC2H3O2

• If base is added, the H from the acid reacts
  with the OH-
• HC2H3O2 OH- ? C2H3O2- H2O
• If acid is added, the acetate ion from the sodium
  acetate is available to neutralize the H
• H C2H3O2- ? HC2H3O2

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Blood Buffer and dissolved CO2

• CO2 dissolves in water to produce carbonic acid,
  H2CO3.
• The other part of the blood buffer system is the
  hydrogen carbonate ion, HCO3-
• Normal blood pH 7.35 to 7.45. Death gt 7.8 or lt 6.8

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Blood Buffer and dissolved CO2

• The amount of CO2, and thus the amount of
  carbonic acid is controlled by respiration in the
  lungs.
• Rapid, deep breathing causes a decrease in CO2 in
  the blood.
• Hyperventilation

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Acid Rain vs. Acid Lakes

• Acid rain can lower the pH of lakes.
• Some lakes in the northeast, Canada, and northern
  Europe have a pH of 4.0. A healthy lake is pH
  6.5.

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Acid Anhydrides Water
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Acid Rain vs. Acid Lakes

• Other lakes get similar amounts of acid rain with
  little pH change.
• The geology of the region is the key to
  maintaining pH.
• Limestone is calcium carbonate, CaCO3.

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Acid Rain vs. Acid Lakes

• Calcium carbonate reacts with carbon dioxide and
  water to produce calcium hydrogen carbonate
• The hydrogen carbonate ions, HCO3-, ions form a
  base that neutralizes acid.

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Chemistry of Antacids

• Stomach acid, HCl, has a pH of about 2.5.
• Enzymes and stomach acid break down complex
  molecules in to smaller molecules.
• Mucous protects the lining of the stomach from
  the digestive power of the stomach acid.

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Chemistry of Antacids

• Antacid Anti-Acids Bases
• Commercial antacids are either hydroxide
  containing bases or carbonate containing bases.
• Hydroxide Antacids have low solubility in saliva.
  They do not dissolve and react until they reach
  the acidic environment of the stomach.

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Chemistry of Antacids

• Carbonate antacids react with HCl to form
  carbonic acid which decomposes to form carbon
  dioxide and water.
• Most carbonates are insoluble in water and have
  great neutralizing power.

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Acid-Base Titrations

• A titration is the process of determining the
  molarity of an acid or a base by using a
  neutralization reaction.
• Standard solution- a solution of known molarity
  used in a titration

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Titration Set Up
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Acid-Base Titrations

• Steps Titration of an acid with a base.
• Measure a volume of unknown acid. A pipet should
  be used.
• Fill a burette with the standard base solution.
• Add indicator to the acid solution.

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Acid-Base Titrations

• Slowly add standard base solution and stir
  constantly until the indicator changes color.
• Determine the final volume of standard solution
  used.

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Stoichiometry

• In a titration, 20.0 mL of HCl of unknown
  molarity reacted with 19.9 mL of 0.100 M NaOH.
  Find the concentration of the acid.
• Balance the equation.
• HCl NaOH ? NaCl H20

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Stoichiometry

• Find the number of moles of standard solution
  used.
• Determine the mole ratio from the balanced
  equation.

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Stoichiometry

• Use the volume of unknown HCl to find the
  molarity.

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Homework

• Read sample problem 4 on pg. 544
• Work Practice Problems 11-13 pg. 546.

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Using a Calculator to Calculate pH

• Some solutions have H3O concentrations that
  are not equal to integral powers of 10.
• These problems require a calculator.
• An estimate of pH can be made to check your
  calculations.
• Remember pH -log H3O, a solution with an
  H3O of 3.4 x 10-5 M has a H3O between 10-4
  M and 10-5 M, and a pH between 4 and 5.

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Using a Calculator to Calculate pH

• To find the exact pH you must use the log key on
  a scientific calculator.
• What is the pH of a solution if the H3O is
  3.40 x 10-5 M?
• pH -log H3O
• pH - log (3.4 x 10-5)
• pH 4.47
• - log ( 3.4 exp - 5 )

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Using a Calculator to Calculate pH

• What is the pH of a 2.60 x 10-3 M solution of
  sodium hydroxide.
• Strong base ? completely dissociates
• Concentration of base OH-
• pH -log (1.00x 10-14 / OH-)
• pH - log (1.00 x 10-14 / 2.60 x 10-3)
• pH 11.4